LIGHT. ]7 



The degree of damage depends, apart from the severity of 

 the frost, on the condition of the leaves and yomig wood, the 

 general health and vigour of the plants, and whether they 

 have been suddenly or gradually deprived of shelter. 



The damage occurs, generally, during the process of thawing 

 after freezing. The more rapidly the plant thaws, the greater 

 will be the damage ; hence it will be greatest on east and 

 south aspects, and smallest on west and north aspects. 



Frost may kill the plant outright, or only the leaves and 

 tender shoots. It may also cause cracks in the bark, as in 

 beech, or in the bark and wood, as in oak. 



2. Lifjht. 



The earth receives light from the sun, the source whence 

 heat is supplied. With the exception of certain low forms, 

 all plants require light to enable them to live and grow, as 

 soon as the available reserve materials have been consumed. 

 Without light carbon dioxide cannot be decomposed by the 

 chlorophyll of the plants. 



During the process of germination light is not required, 

 because the embryo is developed by means of substances 

 deposited in the seed. Similarly, the first starting of vege- 

 tation in spring can take place with a small amount of 

 light, because it is done by means of reserve materials 

 deposited in certain parts of the plant. As soon as these 

 substances, both in germination and the awakening of 

 vegetation in spring, have been consumed, light becomes 

 absolutely necessary for the preparation of new food 

 materials. 



All trees, then, require light for their proper development, 

 but the necessary quantity has its upper and lower limit. 

 Not only too little but also too much light can interfere 

 with the phenomena of growth. Between the maximum and 

 minimum a degree exists which corresponds with the most 

 favourable development of a species, and which represents its 

 normal light requirement. Eegarding the absolute quantity 



